Electronic devices generally produce heat during operation, and thus, heat-dissipating devices or fan assemblies are required. As the demand for heat-dissipation increases, fans must provide enhanced performance.
As shown in FIG. 1, a conventional axial flow fan includes a casing 90, a stator 91, and an impeller 93. The stator 91 is disposed in the casing 90. The impeller 93 includes a rotary shaft 94 inserted in an axial hole of the stator 91 and supported by bearings 92. When the impeller 93 rotates, airflow is directed to an outlet of the casing 90 by blades 95 of the impeller 93. A plurality of ribs 96 are disposed at the outlet of the casing 90. The ribs 96 are substantially a quarter of a circle in cross section, as shown in FIG. 2. The airflow produced by the impeller 93 is blocked by the ribs 96, resulting in different directions of airflow as shown by the arrows in FIG. 2. Noise is produced due to air turbulence near the ribs 96, and the air pressure at the outlet is reduced.